This invention relates generally to color printing, and more particularly to a method for halftoning color images.
Color printing uses a halftoning process to convert continuous tone images to a printable format. For instance, 24 bit/pixel continuous tone image data may be converted into 3 or 4 bit/pixel print data. This allows the use of printing technology that imprints ink in fixed quanta (i.e. one or two droplets per pixel.) With only a few actual colors being printable, the perception of a multitude of color tones is created by the combination of adjacent printed pixels.
Halftoning may also be used for non-printed images, such as those displayed on certain computer screens having pixels unable to display a continuous brightness range.
Halftoning may employ a screen having a matrix of different threshold values. A screen is a data set with each different possible print density value equally represented (or with a controlled unequal distribution for gamma-compensated screens). For monochrome printing, the image data is compared with the screen thresholds at each position, and if the image data exceeds the threshold, a dot is printed; if not, the location remains unprinted. Improved appearance is provided using a pseudo-random stochastic screen having a xe2x80x9cblue noisexe2x80x9d characteristic. Such screens have the threshold values distributed so that adjacent values tend to be very different, and so that any value or limited range of values will tend to be located at positions that are nicely spaced apart on the matrix, avoiding clumping. For example, a 16-by-16 pixel screen having threshold values In the range of 0-255 may be used. This apparently even-butrandom spacing is particularly emphasized at the extremely low and high density values in a blue noise screen.
For color printing, halftoning presents a particular challenge. For dot-on-dot printing, in which printed locations are printed with one or more dots, while nearby pixels are unprinted, a single halftoning screen may be used. For instance, a field of 10% blue would have 10% of locations printed with cyan and magenta ink, while 90% of locations are unprinted. This has the disadvantage of reduced spatial frequency with respect to methods that distribute dots to different locations, and gives the appearance of darker dots, more widely spaced apart, or a xe2x80x9cgrainyxe2x80x9d image. The same applies for clustered dot printing techniques, in which different color dots may be printed adjacent to each other or otherwise clustered to create a multi-dot cluster that reads as an intermediate color. Accordingly, it is preferable to print the individual dots at spaced-apart separate locations, relying on the viewer""s eye to integrate the different color dots into the intended color.
By using different screens having the threshold values arranged differently, the dots will tend not to align with each other. However, with uncorrelated screens, the printed patterns of different colors will tend to be randomly located with respect to each other, leading to some graininess of the image as some dots happen to clump near others or overlap. With two colors, to reduce this, an xe2x80x9cinverted screenxe2x80x9d is used for one of the colors An inverted screen has values equal to the difference between the maximum screen value, and the screen value at the corresponding location on the other screen. Thus, for example, 10% blue is printed by printing cyan dots at all locations where the threshold values of the original screen are 25 or less, and magenta dots at locations of values of 230 and above on the original screen (25 or less on an inverted screen).
Inverted screens are limited in usefulness for high quality printing with 3 or more colors, as color printing normally requires. The third and possibly fourth colors typically must be printed by other means that can generate low frequency visible artifacts. For three and four color systems, a shifted screen approach has been employed to avoid pure dot-on-dot printing for some colors. Not only does this lead to increased graininess of the image, but it often generates unwanted low frequency artifacts that are visible in the printed image. It is also believed possible that moire patterns may be generated.
A more sophisticated halftoning approach for high quality printing of more than two colors has been disclosed in U.S. patent application No. 09/198,024, filed Nov. 23, 1998, to Yao et al., entitled xe2x80x9cColor Printer Halftoning Method,xe2x80x9d the disclosure of which is incorporated herein by reference. While this is effective for very high quality four color printing, it requires significant processing resources to be devoted to the calculations required for halftoning each image.
Accordingly, there is a need for a halftoning process for printing systems employing three or more colors that provides an evenly dispersed pattern of individual drops of different colors, and in which the droplets are dispersed in a high frequency pattern that minimizes unwanted visible image artifacts. In addition, there is a need for a system that does not require significant processor resources for printing.
The present invention overcomes these disadvantages by providing a method of halftoning a color image and particularly by providing a color printing system with a data storage medium containing first, second and third data sets corresponding to first, second and third halftone screens, respectively, having first, second and third screen matrices of threshold values.
The present invention provides a method of halftoning a color image and particularly provides a color printing system with a data storage medium containing a first data set corresponding to a first halftone screen having a first screen matrix of threshold values. The first screen matrix has screen locations corresponding to the matrix of image data elements, and each screen location has a threshold value selected from a range of threshold values. The data storage medium includes a second data set corresponding to a second halftone screen having a second screen matrix of threshold values, each inversely related to the corresponding threshold value of the first screen. The data storage medium includes a third data set corresponding to a third halftone screen having a third screen matrix of threshold values each based on a difference between the first screen value for the corresponding location and a preselected intermediate value in the range of threshold values.
It is a feature of the present invention that the method can employ a halftone screen and its inverted screen for either the cyan or magenta colors and a third screen for yellow that is medially centered with respect to the first screen.